Question

我试图仅从那些在 Pno 3但不在10

中工作的人那里获得下表中的 Essn 值

所以结果应该是"666884444"

我试着像

那样做

SELECT Essn
FROM WORKS_ON 
WHERE Pno = 3 and Pno != 10;

但是无法获得正确的结果

WORKS_ON

---Essn-----------Pno-----------Hours----
"123456789"       "1"           "32.5"
"123456789"       "2"           "7.5"
"666884444"       "3"           "40"
"453453453"       "1"           "20"
"453453453"       "2"           "20"
"333445555"       "2"           "10"
"333445555"       "3"           "10"
"333445555"      "10"           "10"
"333445555"      "20"           "10"
"999887777"      "30"           "30"
"999887777"      "10"           "10"
"987987987"      "10"           "35"
"987987987"      "30"           "5"
"987654321"      "30"           "20"
"987654321"      "20"           "15"
"888665555"      "20"           NULL

Answer 1

你的WHERE-Condition不起作用，因为它只是分别比较每一行的值。有很多方法可以实现这一点，这是我的第一个想法：

SELECT DISTINCT Essn
FROM WORKS_ON 
WHERE Pno = 3 AND Essn NOT IN (SELECT Essn FROM WORKS_ON WHERE Pno = 10);

Answer 2

我认为您可以使用group by和having执行您想要的操作：

SELECT Essn
FROM WORKS_ON 
GROUP BY Essn
HAVING SUM(CASE WHEN Pno = 3 THEN 1 ELSE 0 END) > 0 AND   -- has 3
       SUM(CASE WHEN Pno = 10 THEN 1 ELSE 0 END) = 0;     -- does not have 10

Answer 3

SELECT Essn
FROM WORKS_ON 
WHERE Pno = "3"

这本身只会返回Essn列的Pno，其值为3.

为什么还需要另一个WHERE Pno != 10。

SQL两个WHERE条件为同一列

3 个答案: